It’s a well-worn phrase, frequently deployed by those shaping the UK’s tree-planting strategy, but in the face of an onslaught of pests and diseases, what’s the point of sticking to the mantra of ‘the right tree in the right place’?
‘RIGHT tree in the right place’ is a phrase that has gone viral in forestry. The prefacing of tree-planting pronouncements with these words now appears obligatory, though many who use the diktat remain ignorant of the variations on its theme. Indeed, it was the thinking of those who kick-started contemporary forestry with formation of the Forestry Commission in September 1919. Unencumbered by alien pests and diseases or any fear of climate change, these pioneers were free to follow the science and plant exotic conifers in places where logic said they would thrive.
Compare and contrast that with today’s tree planting, revolving around purist perceptions about native versus non-native trees and attempts second-guess how specific trees will react to a warming climate. Better to study classic texts written by foresters born in the 19th century and among the very best botanists of the day. They knew exactly what they were doing with conifers sourced from as far afield as the Pacific Northwest of the US (Douglas fir), the far-eastern fringes of Asia (Japanese larch) or Europe (Norway spruce and Corsican pine).
CORSICAN PINE – A LOST OPPORTUNITY
Corsican pine (Pinus nigra) is the classic case in point, with a natural distribution centred on southern Europe around the Mediterranean Sea, but subsequently performing well on light, sandy soils the length and breadth of Britain. It was planted extensively by the Forestry Commission in places such as Thetford Chase in the East Anglian Breckland and the Morayshire Culbin sands in Scotland, as well as Dorset in south-west England.
According to H.D. Edlin, writing in the mid-1950s (Trees, Woods and Man), the Forestry Commission was planting 3,000 acres of Corsican pine a year.
With its Mediterranean origins and natural distribution, Corsican pine could have been a prime candidate for thriving in the face of projected UK climate change. However, that bubble burst before current concerns began to heat up via a disease previously called ‘red band needle blight’ due to red-ring symptoms on the pine needles. The disease is now called Dothistroma needle blight after the ascomycete fungal pathogen (Dothistroma septosporum) responsible.
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Another fungal species, called Dothistroma pini, causes an almost identical disease, but Forest Research does not believe this pathogen is present in the UK. However, it occurs in many European countries including France and, given the ongoing situation relating to new pest and disease arrivals, I wouldn’t wager on this pathogen being absent from the UK.
By the late 1990s it was already clear that Corsican pine was essentially finished as a commercial timber tree because of Dothistroma septosporum, but that was before the discussion around climate change and which trees to plant became so animated.
Imagine the situation now, had we managed to exclude Dothistroma needle blight but with the disease still widespread in Europe. Pundits would be all for Corsican pine (and with good reason) as an obvious candidate to face off a warming UK climate. As admirable as that may have been, these plans would almost certainly have been scuppered by the eventual arrival of Dothistroma septosporum (and D. pini) on the back of Britain’s unquenchable thirst for imported European tree-planting material and timber, seemingly oblivious to the consequences.
AN UNFOLDING SAGA FOR SPRUCES
Britain does not have any native spruce species. The two main commercial species which figure in UK forestry are Norway spruce (Picea abies), which as the name suggests is a native of northern and central Europe, and Sitka (Picea sitchensis), native to the western coastal region of North America. Serbian spruce (Picea omorika), native to south-eastern Europe, was introduced to Britain in 1889. Trials showed the tree would thrive on difficult sites, and in 1953 the FC planted 153,000 trees. However, it never became established as a mainstream timber tree.
In contrast, Norway spruce quickly became one of the bedrocks of early UK commercial forestry, though it suffered heavily from wartime tree felling. However, in 1947 there were still 133,000 acres, forming 8 per cent of our high forest cover and 3 per cent of our timber resources. Norway spruce continued to play a large part in 1950s planting programmes, with the FC planting at the rate of 8,000 acres a year. Today, the total stocked woodland area of Norway spruce in Great Britain stands at 61,000 ha, which is 4.66 per cent of the area down to conifers.
In contrast to Corsican pine, Norway spruce is most at home in the moist and mist-shrouded higher elevations of Northern and Central Europe, and by the same token is frost hardy. As such it is best planted in the wetter, western areas of the British Isles, with little found along the drier eastern coastal regions. South of the River Thames you need to walk into the Surrey Hills before finding measurable amounts of Norway spruce.
Looking back, this is no bad thing, as the FC is currently fighting multiple outbreaks of Ips typographus (European spruce bark beetle) in Kent and, more recently, East Sussex.
The fewer Norway spruce trees in this area, the lower the odds of stopping this pest in its tracks before it ‘takes the high road’ to the wide expanses of Sitka in Scotland.
Sitka spruce is the number-one success story in UK commercial softwood forestry, but for how much longer with Ips typographus breeding in south-east England? Sitka has a healthy history in Britain, with 167,000 acres established by 1947, 87 per cent in the safe hands of the FC, which by 1955 was planting it at the rate of 20,000 acres per year.
H.D. Edlin (1956) said: “This puts the Sitka spruce well ahead of any other tree.” And so it turned out, with the total stocked woodland area of Sitka spruce most recently (2021) put at 665,000 ha, 50.84 per cent of the conifer area in Great Britain.
Today, the vast majority of home-grown Sitka is in peaty soils, often on exposed sites in northern England, Wales and Scotland, currently well away from Ips typographus.
However, the threat is clearly there. Susceptibility of Norway spruce, the beetle’s primary host in its native European distribution, is not in doubt, although the potential susceptibility of Sitka spruce is less clear. Better not to find out.
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However, what the recent arrival of Ips typographus clearly demonstrates is how tree selection for the future – in relation to projections and predictions on climate change – could be scuppered overnight due to the unexpected arrival of exotic, alien insect pests and microbial pathogens.
JAPANESE LARCH GOES UP IN SMOKE
Japanese larch (Larix kaempferi) was another conifer (like Corsican pine) with potential to prove immensely useful as a bastion against climate change. Despite its limited native range in the mountains of the central Japanese island of Honshu, it proved to be a versatile conifer around the world.
This naturally light-demanding tree requires at least 1,000 mm of rainfall for good growth and development. Despite a good degree of winter cold hardiness and ability to withstand moderate exposure, these deciduous trees are susceptible to frosts in spring and autumn. Japanese larch is better suited to poor-nutrient-status soils than is its sister species the European larch (Larix decidua).
In 1947 there were 55,000 acres of Japanese larch. In 1956, H.L. Edlin described Japanese larch as a hardy pioneer tree with 9,000 acres being planted annually by the FC. However, during this immediate post-World War II period it was very much the junior to European larch, which still occupied 133,000 acres and ranked number two in conifers in 1947, despite what Edlin described as “heavy fellings during the two world wars”.
Japanese larch went on to become a winner for UK forestry, but its position was essentially lost in 2009 when it was hit by the full-on force of the fungus-like pathogen Phytophthora ramorum. This was an opportunity lost in more ways than one. As Forest Research says of Japanese larch: “A species whose role could have increased in western Britain, especially if planted on mineral soils to diversify spruce forests. But its susceptibility to Phytophthora ramorum has restricted future use.”
European larch and Hybrid larch (Larix x eurolepis) are nowhere near as susceptible to Phytophthora ramorum but have, nevertheless, suffered collateral damage as tree-planting options for commercial softwood forestry. The FC’s forestry statistics do not differentiate between the three different larches, but latest figures put the total stocked woodland area for larch at 126,000 ha.
WHAT’S LEFT – DOUGLAS FIR AND WESTERN HEMLOCK?
UK forestry continued to look on the bright side, reassured by a relatively long list of conifers available to plant as commercial timber trees. At or near the top of the list were Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) – but then along came another alien Phytophthora tree pathogen in 2021. With characteristic aplomb, the FC announced discovery of the somewhat obscure Phytophthora pluvialis, previously found only in western Oregon in the US, and New Zealand. Obscure or not, the pathogen made its intentions clear by making a beeline for Douglas fir and western hemlock.
Back in 1953, Douglas fir plantations covered 38,000 acres and by 1955 the FC was planting 1,500 acres a year. Writing in 1956, Edlin said: “In Douglas fir we have a valuable addition to our timber resources.”
Edlin also marked out western hemlock as worthy of attention as a timber tree, observing how its capacity for natural regeneration showed it was at home in British soil and climatic conditions. 1,000 acres were established in 1953 and by 1956 the FC was planting 360,000 trees per year. Not massive, but a reasonable start for a conifer with a relatively short UK provenance.
However, it appears the following 75 years did not make the most of these species for softwood timber. Forestry Statistics 2021 shows total stocked woodland area of Douglas fir standing at 46,000 ha, the same as that for Corsican pine after a two-decade decline in planting due to Dothistroma needle blight. Western hemlock does not appear by name in the list of conifers.
Phytophthora pluvialis has now been identified on commercial conifers right up the western maritime flank of the British Isles. From south to north, outbreak sites are in Cornwall and Devon, all four corners of Wales, Cumbria and western Scotland in Argyll and Ross and Cromarty. It has been confirmed as the causal agent of resinous stem cankers on western hemlock. Pictures of this new stem canker disease released by the FC and Forest Research show it could do to western hemlock, and perhaps Douglas fir, what Phytophthora ramorum has already done to Japanese larch. Loss of either tree from UK commercial forestry would be serious, with loss of Douglas being the bigger blow.
FRAXINUS IN ASHES
Plantation conifers are not the only trees being systematically picked off by pests and pathogens, throwing future tree-planting plans into chaos. Until 2011, common ash could reasonably have been described as the ‘right tree for almost any place’ in the UK. But one year later, chalara ash dieback (Hymenoscyphus fraxineus) was found spreading at breakneck speed throughout the wider environment. Within just a few years, huge swathes of trees were being felled across the country. It did not matter whether ash was growing in the relatively dry soils of East Anglia, on wet London clay or on the limestone slopes of the east midlands and Yorkshire. Disease was spreading and trees were coming down. All planting of Fraxinus excelsior effectively ceased in 2012, with forest nurseries lifting and torching millions of seedling trees.
SWEET CHESTNUTS IN THE FIRING LINE
Sweet chestnut (Castanea sativa) was prominent amongst the broadleaves recommended as a replacement for common ash, but was already at the beginning of some potentially big trouble in 2011 with the discovery of chestnut blight (Cryphonectria parasitica) in Warwickshire. It has since been found on scores of sites across a wide area of England.
Compounding this problem is Phytopththora ramorum, for which Castanea sativa appears to becoming a significant host – and a sporulation host species to boot.
Susceptibility of sweet chestnut to Phytophthora ramorum became apparent soon after the first finding of the disease in 2004, but only when trees were growing in close proximity to powerful sporulation hosts like Rhododendron ponticum.
However, since 2015 the disease has been found on sweet chestnut trees away from R. ponticum, pointing toward tree-to-tree infection within Castanea sativa stands. The potential for damage by this pair of pathogens, together with the additional finding of oriental chestnut gall wasp (Dryocosmus kuriphilus) in Kent in 2015, and its subsequent rapid spread across the south of England, could jeopardize future planting potential of sweet chestnut. Especially since damage caused by this pest renders trees more susceptible to infection by Cryphonectria parasitica.
NOT THE END OF A SAD STORY FOR CONIFERS
Problems also exist for other tree species outside the contemporary forestry arena.
Lawson’s cypress (Chamaecyparis lawsoniana) is a back-row conifer that may well come onto the UK commercial forestry agenda in future. One reason why Lawson’s cypress hardly registers in UK commercial forestry has more to do with timber processing than tree planting. Apparently, the nature of the bark is not compatible with sawmilling machinery used here. Be that as it may, Lawson’s cypress has already proved fatally susceptible to Phytophthora lateralis, found for the first time in 2010.
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By no stretch of the imagination is common juniper a timber tree, but as one of only three British native conifers it is a vital component of the wider landscape. It was already in steep decline before Phytophthora austrocedri appeared in 2011 and, a decade later, the fungus-like pathogen looks likely to deliver a fatal blow to wild Juniperus communis in the UK.
A CANKEROUS DISEASE OF HORSE CHESTNUT
There was a time not so long ago when white-flowering horse chestnut (Aesculus hippocastanum) was among the most popular trees for landscape and amenity planting.
The species had led a comparatively charmed life since it was introduced from Turkey during the reign of Queen Elizabeth I.
However, almost half a millennium later, during the ‘noughties’ (2000-2009), horse chestnut leaf miner (Cameraria ohridella) and the plant pathogenic bacterium responsible for bacterial bleeding canker (Pseudomonas syringae pv. aesculi) arrived in quick succession. Horse chestnut leaf miner quickly developed into a supreme nuisance pest, ruining the canopies of these majestic trees.
Horse chestnut leaf miner has not proved fatal, but the same cannot be said of bacterial bleeding canker, which swept through the horse chestnut population, killing trees across the whole country. Red-flowering horse chestnut (Aesculus x carnea) proved to be even more susceptible than white-flowering horse chestnut. Some local authorities continued to plant horse chestnut, but most realised it was a complete waste of time. However, the option to plant horse chestnut was quickly closed off as nurseries removed the trees from their species selection.
‘GUNPOWDER TREE’ GOING OUT WITH A BANG
Common alder (Alnus glutinosa) was never a mainstream timber tree, although it once provided huge amounts of charcoal for the gunpowder-making industry, before being hit by the explosive effects of yet another Phytophthora pathogen – Phytophthora alni.
Other species of alder, including Italian alder (Alnus cordata) and grey alder (Alnus incana), are also susceptible to infection.
The peculiar thing about Phytophthora alni is how it came to be in the UK. It is neither alien nor native in the classic context, but originated here relatively recently (first found in 1993) through ‘inter-specific’ hybridization. I seem to recall mycologists originally suggesting it was the result of a cross between Phytophthora infestans (the potato blight pathogen) and another unknown Phytophthora. Latest reports say it may have Phytophthora cambivora as an ancestor.
Be that as it may, Phytophthora alni is most prevalent in south-east England, but is also causing significant damage in the borders region of Wales and on Scottish river systems.
Like most other Phytophthora diseases affecting trees, this one is fatal and has the capacity to change the tree composition of riverbanks and riparian strips. Estimates suggest at least one fifth of alder trees in Great Britain are now affected.
‘HEARTS OF OAK’ AT RISK
Finally, even pedunculate oak (Quercus robur) and sessile oak (Quercus petraea) cannot escape the depredations of alien insect pests. Oak processionary moth or OPM (Thaumetopoea processionea) arrived in London in 2005 and has since made slow but steady progress to affect oak trees in every London Borough and large parts of the surrounding Home Counties.
Around 10 years ago, when the plant health authorities finally admitted OPM eradication was no longer an option and responsibility for control (including costs) was essentially passed from government to stakeholders and landowners, many started to reassess their commitment to the national flagship tree.
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